1. Field of the Invention
The invention relates to polymeric compositions useful as insulation for wire and cable products used for underhood automotive applications or 125.degree. C. rated appliance wire applications. More specifically, the compositions of the invention are ethylene-vinyl ester copolymer formulations with improved irradiation crosslinkability.
2. Detailed Description of the Prior Art
Polymeric compositions exhibiting a balance of physical properties, processability and flame retardance comprised of crosslinkable ethylene-vinyl acetate copolymers, silanes and hydrated inorganic filler(s) are known and have found wide acceptance in the wire and cable industry. Such compositions are disclosed in U.S. Pat. Nos. 3,832,326 and 3,922,442 to North, et al., and U.S. Pat. Nos. 4,349,605 and 4,381,362 to Biggs, et al. Besides the polymer, silane and hydrated filler, the formulations typically also contain other additives such as stabilizers, lubricants, antioxidants and the like.
To achieve useful wire and cable insulation compositions having the necessary balance of physical properties and thermal and chemical resistance, it is necessary that the compositions be crosslinked. This can be accomplished using chemical crosslinking agents, typically organic peroxides, or by exposing the composition to ionizing radiation.
Irradiation crosslinking has had limited commercial acceptance in the wire and cable industry. Contributing to this lack of commercial acceptance is the reduced crosslinking efficiency obtained with stabilized formulations. It is well known that the commonly used phenolic antioxidants reduce radiation crosslinking efficiency to generally unacceptable levels and, if the necessary degree of crosslinking is to be achieved in formulations containing such stabilizers, it is necessary to include a promotor. Useful promotors are typically multifunctional compounds, e.g., multifunctional acrylates and methacrylates.
It would be highly advantageous if wire and cable formulations were available which could be radiation crosslinked to achieve acceptable levels of cure without the use of multifunctional promoters. It would be even more advantageous if these compositions had acceptable balance of properties making them suitable for low tension primary cable applications. These and other objectives are realized with the improved compositions of the invention which incorporate a zinc salt of a mercaptobenzimidazole compound.
Benzimidazole compounds are known stabilizers for thermoplastic resins such as polyethylenes and polypropylene. U.S. Pat. No. 3,218,276 discloses the use of alkyl benzimidazoles to stabilize fiber-forming polyolefins. Polypropylene fiber-forming compositions containing 0.2 to 2.0 percent benzimidazole with other conventional additives are disclosed.
U.S. Pat. No. 2,997,456 teaches the use of metallic mercaptobenzimidazole compounds as stabilizers for polymers of 1-olefins, primarily polypropylene, to protect against molecular degradation under conditions of elevated temperature and/or mechanical working and zinc mercaptobenzimidazole is specifically mentioned.
The combination of hindered phenols with various zinc salts of mercapto compounds to stabilize polyolefins is taught in U.S. Pat. Nos. 4,260,661; 4,693,937; 4,797,323 and 4,824,883. Other references which disclose benzimidazole stabilizers for polymeric materials include U.S. Pat. Nos. 4,459,380; 4,808,643 and 5,196,462.